The present invention relates generally to a position measurement system, particularly, although not exclusively, to a method and apparatus for determining the position of scanning printer carriages in inkjet printer devices.
Inkjet printer devices generally incorporate one or more inkjet cartridges, often called xe2x80x9cpensxe2x80x9d, which shoot drops of ink onto a page or sheet of print media. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, both assigned to the present assignee, Hewlett-Packard Company. The pens are usually mounted on a carriage, which is arranged to scan across a slider rod that traverses a print zone, in which a sheet of print media may be located. As the carriage traverses the print zone, the pens print a series of individual drops of ink on the print media forming a band or xe2x80x9cswathxe2x80x9d of an image, such as a picture, chart or text. The print media is subsequently moved relative to the carriage, so that a further swath may be printed adjacent to the earlier swath. By a repetition of this process, a complete printed page may be produced in an incremental manner.
In order to generate high quality printed output, it is necessary that the ink drops from the individual pens are accurately applied to the print media. This is made possible by accurately measuring the position of the carriage as it traverses the print media. This is generally achieved using an encoder strip or codestrip, which is arranged parallel to the scan direction of the carriage. Such a codestrip is usually made from a plastics material such as Mylar(trademark), upon which a series of graduations or marks are recorded. The graduations, which may be recorded using a laser plotter, give rise to local variations in the properties (such as optical properties) of the codestrip. An optical sensor mounted on the carriage, may be used to sense the optical variations in the codestrip as the carriage moves relative to it. The output of the sensor may be used by a microprocessor associated with the printer device to generate position and speed information relating to the carriage.
However, the carriage support and guide subsystems are prone to manufacturing imperfections. One common such imperfection is a lack of straightness. Thus, in existing printers of this type, the carriage has a tendency to make small rotations about a given axis, as it traverses the scan axis; for example its vertical axis, which is often known as the xe2x80x9cZxe2x80x9d axis. This has the effect of causing the actual position across the scan axis of the printheads supported in the carriage to vary from their measured positions. This variation may give rise to a systematic error in the position in which ink dots are printed on the print medium. For example, because different coloured printheads are usually spaced apart from one another in the direction of the scan axis, the degree to which each printhead is rotated about the xe2x80x9cZxe2x80x9d axis when it passes over a given point in the print zone may be different Where the degree of rotation between printheads is different, inks drops of different colours that should be printed in the same position in an image may be printed in different positions. Where a compound colour is being printed, the result may be a colour that has a hue which varies along the scan axis. This hue shift may give rise to a noticeable print quality defect commonly know as xe2x80x9cvertical bandingxe2x80x9d.
It would therefore be desirable to provide a hard copy device and method, which addresses the problems of the prior art
According to a first aspect of the present invention there is provided an inkjet device having a carriage arranged to support a printhead and to scan across a print zone, the device comprising first and second codestrips traversing the print zone, the device being adapted to generate first and second carriage position information from the first and second codestrips respectively and being further arranged to determine the position along the scan axis of at least part of the printhead from an average of the first and second carriage position information.
By measuring the position of the carriage or printhead relative to the scan axis relative to more than one codestrip, position and orientation information relating to the carriage or printhead may be determined. Thus, the position of a precise location or part of the printhead or carriage may be known relative to the scan axis, for example. Thus, in the event that the orientation of the printhead or carriage alters as it crosses the scan axis, for example due to imperfections associated with the straightness of the scan axis, this may be compensated for. This may be achieved by changing the timing of the firing of the nozzles of the printhead. This technique may help to reduce drop placement errors in the ejected nozzles. In this manner, print defects may be reduced.
In one embodiment, rotation of a printer carriage about its vertical axis (Z axis) is compensated for. In another embodiment, rotation of a printer carriage about its horizontal axis (Y axis), perpendicular to the scan axis, is compensated for. In each of these embodiments first and second codestrips may be used, which are spaced apart in a third axis; the third axis being orthogonal to both the scan axis and the axis about which the rotation to be compensated for occurs.
In one embodiment of the present invention, two codestrips are located spaced apart in the Y axis (the media feed direction), each at a relatively large distance from the printhead. This offers the advantage of avoiding cluttering the generally crowded print zone area with a codestrip. At the same time, by generating an averaged or virtual position of a printhead of the printer, from the two codestrips, the position of the printhead may be accurately determined, even if in the carriage is subject to changes in orientation whilst traversing the scan axis. Conventionally, designers of such systems have attempted to reduce the distance between the codestrip and the printheads. This is because the greater the distance that separates a single codestrip and the printheads, the greater may be the difference in measured and actual position of the printheads when the carriage orientation changes. Consequently, the greater the drop placement error may be. Therefore, the placement of a single codestrip has traditionally been made as close as practicable to the printheads. Thus, it has been a trade off between accepting a degree of drop placement error and design cost. Here the design cost may be in terms of improving the quality of the scan axis in order to reduce imperfections in its straightness for example, and/or attempting to design the print zone to permit the codestrip to be located as close as possible to the prinheads.
In certain embodiments of the invention, one codestrip is located on either side, in the Y axis direction, of the printhead(s). The distance separating each codestrip from the printhead(s) in the Y axis is the same. In this manner, the virtual position signal for the printhead(s) may be a simple average of the signals derived from the two codestrips. This gives rise to the advantage of requiring only a simple computation to determine accurately the position of, for example, the centre of the printhead(s) in the Y axis.
In other embodiments of the invention, the distances separating each codestrip from the printhead(s) may be different. In such embodiments, the virtual position signal for the printhead(s) may be a weighted average of the signals derived from the two codestrips; with the weighting being dependent upon the relative distances that the two codestrips are separated from printheads. This gives rise to the advantage of giving design flexibility to the design of the hard copy device, allowing the relatively unconstrained placement of the codestrips relative to the print zone. It will thus be understood that the average of the first and second carriage position information, be this a weighted, simple or other form of average, may be viewed as a composite of the first and second carriage position information.
In other embodiments of the invention, more than one virtual position signal may be generated from the two codestrips. These may each have a different weighting of the two signals generated from the two codestrips. Unlike single codestrip systems, this gives rise to the advantage of being able to determine the position along the scan axis of two or more points or areas of the carriage or printhead(s) at the same time, where those points occupy different locations in the in the media feed direction. In the case where large printheads are used this may be especially beneficial since even a small rotation of large printhead may cause appreciably different drop placement positions between nozzles in different positions in the printhead(s); and thus appreciable drop placement errors. In this manner, according to such embodiments, the firing of different groups of nozzles or indeed individual nozzles may be independently controlled in dependence upon their detected positions.
The present invention also extends to the method corresponding to the apparatus. Furthermore, the present invention also extends to a computer program and a processor, arranged to implement the method of the present invention. Further aspects of the invention will be apparent from the appended claims.